Dual-route models of response selection postulate two processes that contribute to SRC effects: automatic activation of the corresponding response via the direct route and efficiency of translation to the assigned response via the indirect route (e.g., De Jong et al., 1994
; Proctor & Vu, 2006
). If it is assumed that automatic activation of the corresponding response becomes stronger with experience, in this case with age, then older adults should show larger SRC and Simon effects in the pure conditions, as typically found (Proctor et al., 2005
). Similarly, if it is assumed that S-R translation becomes more efficient for particular S-R pairings as a function of experience, then the translation benefit for the compatible location mapping in the indirect route should also increase with age. Consequently, the benefit for corresponding responses should be relatively larger with age when location is relevant than when it is irrelevant because the compatible location mapping gains in both translation efficiency and stronger automatic activation. In other words, compared to the effects shown by younger adults, the SRC effect for older adults should be relatively larger than the Simon effect.
In Experiments 1 and 2, with pure compatible mappings of stimulus locations to responses, older adults were slower than younger adults, but with pure incompatible mappings the difference between the two age groups was much larger. For the pure Simon task, older adults were also slower than younger adults when stimulus and response locations corresponded but only slightly more so when they did not. Thus, as predicted from dual-route models, the cost of noncorrespondence between S-R locations for older adults was larger when stimulus location was relevant than when it was not.
When compatible and incompatible mappings are mixed, or when a location-relevant task is mixed with a location-irrelevant task, younger adults typically show elimination of the SRC effect for location-relevant trials (see Vu & Proctor, 2004
). This elimination is often attributed to suppression of the direct response-selection route. If older adults are less able than younger adults to suppress this route, then mixing should not result in as much reduction of the SRC effect for older adults. Yet, in Experiment 1, the SRC effect was reduced more for older than younger adults when compatible and incompatible spatial mappings were mixed. Similarly, in Experiment 2, the SRC effect was reduced by a mixed location-irrelevant task as much for older adults as for younger adults. These results suggest that the decrement in performance with a pure incompatible mapping for older adults is not due primarily to a deficiency in overcoming automatic response tendencies.
The Simon effect is typically attributed to activation of the corresponding response through the direct response-selection route. As mentioned above, the Simon effect for the pure condition was larger for older than younger adults. Mixing showed a qualitatively similar pattern of influence on the Simon effect for younger and older adults. When the intermixed location-relevant trials were compatibly mapped, the Simon effect increased for both RT and PE, with the increase in the Simon effect for RT being slightly larger for older than younger adults. When the intermixed location-relevant trials were incompatibly mapped, the Simon effect reversed in both RT and PE, with the reversal being larger for the younger adults only in the PE data. Moreover, if older adults had more difficulty in suppressing automatic activation, then they should show qualitative differences in the RT distribution functions from those shown by younger adults. Yet, for both the pure and mixed conditions of Experiment 2, the RT distributions for the Simon task were mainly shifted to longer times for the older adults. These data for the Simon effect thus also show relatively little indication that older adults are deficient in suppressing the long-term associations of the direct route under mixed task conditions, in agreement with the results for the location-relevant trials.
The cost of mixing spatial tasks is also usually greater for older than younger adults (e.g., Meiran et al., 2001
). In Experiment 1, the cost on RT of mixing compatible and incompatible spatial mappings was 204 ms larger for older than younger adults. In Experiment 2, when a location-relevant task with a single mapping was mixed with a location-irrelevant task, the additional mixing cost for older adults compared to younger adults was only 65 ms, and this additional cost was even less for the location-irrelevant task (20 ms). Thus, the additional mixing cost for older adults was much larger when the mixed tasks were both spatial and their S-R mappings conflicted. In Experiment 1, the color of the stimulus had to be identified when compatible and incompatible mappings were mixed but not when they were presented in pure blocks. Thus, the greater mixing cost for older adults could be attributed to this difference in identification requirements rather than to additional difficulty in response selection. If such were the case, though, then in Experiment 2 older adults should have shown a similar additional mixing cost for the location-relevant task because identification of stimulus color was required only for the mixed-tasks condition. That the additional age-related mixing cost was considerably smaller in Experiment 2 than in Experiment 1 therefore provides evidence that majority of the additional cost in Experiment 1 was not due to the difference in color-processing requirements for the pure and mixed mapping conditions.
Previous studies have found that older adults do not show a deficit in specific switch costs when the task on the current trial differs from that on the previous trial. Examples include studies by Reimers and Maylor (2005)
for tasks requiring categorization of faces by gender or emotion and Eppinger et al. (2007)
for switching between meaning of color word or ink color in a Stroop color task. Consistent with the prior findings, the sequential analyses of the mixed conditions in the present study showed similar patterns of repetition benefits and switch costs for younger and older adults. The main difference was that older adults benefited more than younger adults from complete repetition of the prior trial for the location-relevant task in both Experiments 1 and 2. For the location-irrelevant task of Experiment 2, the older adults showed no statistically reliable difference from the younger adults in sequential-effect magnitudes. Thus, the main additional decrement in performance for older adults when tasks were mixed was in the overall mixing costs and not the specific switch costs. These results imply that the deficiency shown by older adults is primarily in the ability to maintain spatial task sets with conflicting response mappings in working memory rather than in the ability to reconfigure the S-R mapping, or suppress/release the direct route, on a trial-to-trial basis.
Our results imply that activation of a left/right response code through the direct route is not particularly problematic for older adults. Although the Simon effect was larger for older than younger adults, the absolute increase in effect size and the costs of mixing Simon trials with location-relevant trials were relatively small. Thus, the processes involved in preventing irrelevant information from having much influence on performance do not seem to be as impaired in older adults as those involved in intentional response selection. This conclusion is consistent with the finding that although older adults’ show little deficiency in the ability to prepare subsets of responses, they are slowed in doing so for subsets that require effortful selection (Proctor et al., 2006
That the cost of noncorrespondence becomes relatively greater for older adults as the need to attend to location increases is in agreement with our earlier findings comparing the standard Simon task with the accessory Simon task (Proctor et al., 2005
). For this latter task, the irrelevant location information is usually conveyed by a left or right auditory tone separate from the visual stimulus that conveys the relevant color information. Older adults showed a Simon effect of larger absolute size than that of younger adults for the standard Simon task in which the location information was integrated with the color information, but they did not in accessory stimulus versions of the task, including one in which the accessory was a left or right visual stimulus. Thus, when there is no need to attend to a stimulus’s location, its influence on performance is similar for older and younger adults. When location must be attended to evaluate a relevant stimulus dimension other than location, as in the Simon task, the irrelevant location information is more problematic for older than younger adults. Older adults have still more difficulty when stimulus location not only must be attended but also is relevant and mapped incompatibly to responses, and the difficulty becomes extreme when competing responses are mapped to each stimulus location. These results suggest that automatic processing of the location information is not the cause of the additional difficulty for older adults. When attention is directed to the location information for processing of the relevant dimension, older adults experience more difficulty when the activated location code conflicts with that of the assigned response.
The difficulty that older adults experienced in our Experiment 1 with mixed mappings that applied to the same set of stimulus locations is in agreement with results of Meiran et al. (2001)
. In their Experiment 1, stimuli could occur in one of four quadrants of a square, and two response keys were arrayed along one diagonal. A cue indicated whether subjects were to respond based on the horizontal or vertical location of the stimulus. The irrelevant dimension could be congruent or incongruent with the cued dimension. Their results showed that age had a larger influence on the overall mixing cost than on the congruency effects, and they concluded that the selective aspect of attention reflected in the congruency effects was less affected by aging than were the intentional response-selection processes. Taken together, the results with spatial tasks converge on Mayr and Liebscher’s (2001)
conclusion that global mixing costs “seem to be particularly large when demands in terms of ‘keeping competing mental sets apart’ are high” (p. 47).